WIP: draft incomplete keep rule for keeping most recent common ancestor

pruning/keep_most_recent_common_ancestor.go

@@ -0,0 +1,29 @@
+package pruning
+
+import "sort"
+
+type KeepMostRecentCommonAncestor struct {
+	_opaque struct{}
+}
+
+func NewKeepMostRecentCommonAncestor() *KeepMostRecentCommonAncestor {
+	return &KeepMostRecentCommonAncestor{}
+}
+
+func (k *KeepMostRecentCommonAncestor) KeepRule(snaps []Snapshot) (destroyList []Snapshot) {
+	var bothSides []Snapshot
+	for _, s := range snaps {
+		if s.PresentOnBothSides() {
+			bothSides = append(bothSides, s)
+		}
+	}
+	if len(bothSides) == 0 {
+		return []Snapshot{}
+	}
+
+	sort.Slice(bothSides, func(i, j int) bool {
+		return bothSides[i].Date().After(bothSides[j].Date())
+	})
+
+	return []Snapshot{bothSides[0]}
+}